Controlling the Morphology of Au–Pd Heterodimer Nanoparticles by Surface Ligands

Abstract

Controlling the morphology of noble-metal nanoparticles is mandatory to tune specific properties such as catalytic and optical behavior. Heterodimers consisting of two noble metals have been synthesized, so far mostly in aqueous media using selective surfactants or chemical etching strategies. We report a facile synthesis for Au@Pd and Pd@Au heterodimer nanoparticles (NPs) with morphologies ranging from segregated domains (heteroparticles) to core–shell structures by applying a seed-mediated growth process with Au and Pd seed nanoparticles in 1-octadecene (ODE), which is a high-boiling organic solvent. The as-synthesized oleylamine (OAm) functionalized Au NPs led to the formation of OAm-Au@Pd heteroparticles with a “windmill” morphology, having an Au core and Pd “blades”. The multiply twinned structure of the Au seed particles (⌀ ≈ 9–11 nm) is associated with a reduced barrier for heterogeneous nucleation. This leads to island growth of bimetallic Au@Pd heteroparticles with less-regular morphologies. The reaction process can be controlled by tuning the surface chemistry with organic ligands. Functionalization of Au NPs (Ø ≈ 9–11 nm) with 1-octadecanethiol (ODT) led to the formation of ODT-Au@Pd NPs with a closed Pd shell through a strong ligand–metal binding, which is accompanied by a redistribution of the electron density. Experiments with varied Pd content revealed surface epitaxial growth of Pd on Au. For OAm-Pd and ODT-Pd seed particles, faceted, Au-rich domain NPs and impeded core–shell NPs were obtained, respectively. This is related to the high surface energy of the small Pd seed particles (⌀ ≈ 5–7 nm). The metal distribution of all bimetallic NPs was analyzed by extended (aberration-corrected) transmission electron microscopy (HR-TEM, HAADF-STEM, EDX mapping, ED). The Au and Pd NPs, as well as the ODT-Au@Pd and OAm-Pd@Au heteroparticles, catalyze the reduction of 4-nitrophenol to 4-aminophenol with borohydride. The catalytic activity is dictated by the particle structure. OAm-Au@Pd heteroparticles with faceted Au domains had the highest activity because of a mixed Au–Pd surface structure, while ODT-Au@Pd NPs, where the active Au core is covered by a Pd shell, had the lowest activity.

Synthesis of Au, Ag, and Au–Ag Bimetallic Nanoparticles Using Pulicaria undulata Extract and Their Catalytic Activity for the Reduction of 4-Nitrophenol

“…The reduction of 4-NPh to 4-aminophenol (4-APh) in the presence of NaBH 4 is thermodynamically favorable. However, metal catalysts are needed to overcome the kinetic barrier [ 43 , 44 ] to transfer the electron from the BH 4 − donor to the 4-NPh acceptor. The catalytic reactions were performed in a standard quartz cuvette (3 mL volume, 10 mm diameter), as described elsewhere [ 18 ].…”

Well-Defined Nanoparticle Electrocatalysts for the Refinement of Theory

“…Additionally, because small metal NPs are inherently unstable, they are usually covered with a monolayer or sub-monolayer of monomeric or polymeric ligands. In addition to providing stability, the ligands or capping agents used during NP synthesis can also be used to control the structure and composition of the resulting NP; − however, the ligands themselves can alter the electrocatalytic activity of the underlying metal NP. − Keeping these limitations in mind, a brief introduction to the principal synthetic techniques currently used for preparing metal nanoscale electrocatalysts with a high level of synthetic control is provided next.…”

Controlled Synthesis of Intermetallic Au2Bi Nanocrystals and Au2Bi/Bi Hetero‐Nanocrystals with Promoted Electrocatalytic CO2 Reduction Properties

“…No Bi NPs could be found in the reaction solution by heating Bi 3+ (0.02 mmol) in the mixture of AA (0.2 mmol) and OAm (10 mL) for the first 5 min, indicating that the homogeneous nucleation of Bi NPs faced high kinetic barriers and required higher reaction temperatures and/or longer reaction time. On the contrary, as revealed by time‐sequential XRD patterns and TEM images (Figures 3a–c and S3, Supporting Information), small Au NPs were formed once the Au precursor (0.02 mmol, Au/Bi=1 : 1) was added to the hot mixture due to the rapid reduction of Au III to Au 0 by AA and OAm [29–31] . Once formed, such Au NPs could act as heterogeneous nucleation centers to lower the kinetic barriers to reduce Bi 3+ species.…”

“…On the contrary, as revealed by time-sequential XRD patterns and TEM images (Figures 3a-c and S3, Supporting Information), small Au NPs were formed once the Au precursor (0.02 mmol, Au/Bi = 1 : 1) was added to the hot mixture due to the rapid reduction of Au III to Au 0 by AA and OAm. [29][30][31] Once formed, such Au NPs could act as heterogeneous nucleation centers to lower the kinetic barriers to reduce Bi 3 + species. Afterwards, Au NPs were gradually depleted, and Au 2 Bi NPs emerged as the reaction continued (Figure 3d,e).…”